A related-art image forming apparatus has a plurality of functions. JP-A-11-177731 (especially, paragraph [0066]) discloses an image forming apparatus has a normal power mode in which a plurality of functions are available and a low power consumption mode in which power consumption is reduced by deactivating the functions when the functions are not utilized. In the normal power mode, power is supplied to the entire apparatus from a main power source. In the low power consumption mode, power is supplied from the main power source to only a detection circuit for detecting an external trigger for restoring the apparatus to the normal power mode, such as a circuit access and an operation. When the low power consumption mode is selected, the power supplied to the entire apparatus is reduced to the power supplied solely to the detection circuit. Hence, the power consumed by the apparatus is decreased.
However, in the above image forming apparatus, power is supplied from the main power source in any of the modes; namely, the low power consumption mode and the normal power mode. The main power source generates large electric power used for operating the entire apparatus, and therefore the main power source itself consumes power. When the low power consumption mode is selected, only the detection circuit operates, and the other circuits are deactivated. However, power is supplied to the detection circuit by the main power source, and hence power consumption of the main power source remains the same. Therefore, even in the low power consumption mode, the power consumption may not sufficiently reduced.